1. Field of Invention
The present invention pertains to ultrasonic transducer for the determination of the acoustic power of an ultrasonic field focused in a focus.
2. Description of the Prior Art
Focused ultrasonic fields are employed, for example, in medicine, to break up concretions present in the body, e.g. kidney stones. Such procedure is advantageous for the avoidance of surgical interventions including the introduction of probes into the patient's body and for the prevention of dangerous infections.
Equipment for breaking up concretions without direct contact is known from the EP-A2 No. 0 133 665, where the approximately plane ultrasonic wave produced in a shock tube is focused on the target area with an acoustic lens or an acoustic system of lenses. A liquid coupling medium is located between the lens or the system of lenses and a copper membrane serves as an ultrasonic radiator. The shock tube can be aimed at the concretion with the aid of a suitable holder so that the focal point of the lens on the image side lies in the concretion. The lens can also be shifted along the axis of the shock tube with the aid of a fine adjustment means for the exact location of the focal point.
So-called miniature hydrophones are used for measuring an ultrasonic field produced by an ultrasonic radiator in a sound carrying medium, e.g. water. The three-dimensional distribution of the acoustic pressure amplitude of the ultrasonic field produced by the transmitter radiator is determined by measuring the acoustic pressure existing in the measuring tank at various sites with the hydrophone.
If the spatial distribution of the amplitude and the direction of the propagation of the sonic field in a cross-sectional area of the path of rays is known, the flow of energy through this plane can be determine therefrom.
In a known broad-band miniature hydrophone, a piezoactive foil of electrically insulated polyvinylidene fluoride PVDF with a thickness of 25 .mu.m (micrometers) and equipped with electrodes is stretched across the front area of a refined steel tube. The diameter of the foil is about 1 mm. A platinum wire that is connected to a coaxial cable is located on the inside of the foil. This platinum wire is supported by a backing with which the inside of the refined steel tube is filled. The outside of the foil is in electrical contact with the refined steel tube. The outside of the foil is in electrical contact with the refined steel tube and connected to the shielding of the coaxial cable (Ultrasonics, September 1981, pp. 213 to 216).
Also known is a membrane hydrophone (Ultrasonics, May 1980, pp. 123 to 126), in which a polyvinylidene fluoride PVDF membrane with a thickness of 25 .mu.m and a diameter of 100 mm is stretched between two metal rings. In the center of the membrane there is the piezoelectrically active area, which is equipped with electrodes and has a diameter of 4 mm, for example.
Several mutually independent measurements must be made in a cross-sectional area of the path of rays to determine the acoustic power of a focused ultrasonic field with the aid of such hydrophones. This can be accomplished, e.g., by performing several sequential measurements with one hydrophone. But such a procedure is time consumming and cannot be performed, for example, during the use of a medical ultrasonic device, such as a lithotripter. The time expended for the measurement can be reduced when a several hydrophones are arranged in the cross-sectional area that can be read approximately simultaneously, but this requires large structural expenses. Besides, the interferences associated with the ultrasonic field in this type of structure are not negligible.